The invention relates generally to the field of radioisotope cameras and, more particularly, to circuitry for automatically correcting intrinsic nonuniformities in the displayed image.
Diagnosis of tumors and other diseased tissue has been greatly facilitated by the advent of nuclear medicine. For example, small amounts of radioisotopes, after being administered to a patient, concentrate differently in diseased and healthy tissues. The different concentration of radiation, usually gamma rays, emitted by the healthy and diseased tissues are thus distinct and can be detected. The machines used to detect the radiation usually utilize a collimator to direct or transmit radiation to a scintillation system which changes the radiation to visible light during a scintillation. Photomultiplier tubes detect the light and various means are used to locate the scintillations in the scintillator and, thus, indirectly to find the tumor or other irregularity in the patient.
Radiation imaging devices include dynamic scanning machines and static radiation cameras. Both devices have inherent limitations. The scanners move slowly over the patient and are therefore considered to have better resolution and field uniformity. However, because scanners take a relatively long time to detect the radiation, they create some patient discomfort. A static imaging device, on the other hand, is relatively fast because it takes a single stationary picture. While faster than the scanner, the radiation camera does not yield as good resolution and field uniformity as the scanner. Resolution is used herein to mean the ability of the machine to distinguish two spaced points or line sources of radiation.
An example of an early radiation camera is shown in U.S. Pat. No. 3,011,057 to Anger and U.S. Pat. No. 3,911,278 to Stout, the disclosures of which are incorporated by reference. The scintillation camera uses an array of parallel photomultiplier tubes spaced away from the scintillation crystal assembly so that the tubes view overlapping areas of the crystal. Thus, a scintillation in the crystal is detected and converted to an electrical pulse by several of the tubes at once. The electrical output of the photomultiplier tubes is amplified and algebraically manipulated by suitable circuitry and discriminated for proper pulse height for gamma radiation to produce an analog intensity signal (z axis) and a pair of x,y deflection voltages which are applied to a storage oscilloscope or nonstorage oscilloscope used in conjunction with photographic film. The oscilloscope reproduces each scintillation as a bright spot on the cathode ray tube screen located in accordance with the x,y voltages. Each scintillation event is separately displayed so that over a period of time, an image of all of the scintillations which have occurred in the scintillation crystal over that period of time is obtained. Using conventional collimator techniques involving apertured lead shields, the image on the screen will show an image of the actual distribution of the radiation in the organ being viewed by the camera.
In the 3,011,057 patent, the spacing of the tubes from the scintillators causes the failure of some photons to be detected by the photomultiplier tubes and a loss in resolution results. If this loss in resolution caused by spacing the tubes away from the crystal can be avoided, static radiation cameras can yield results which are more comparable to those of the scanning devices.
The present invention seeks to overcome the disadvantage of the prior art static camera devices. In particular, this invention relates to improving the uniformity of scintillation cameras. Prior scintillation cameras use a hexagonal array of parallel photomultiplier tubes placed directly against or spaced from the scintillation crystal assembly so that there is overlapping in the field of view of the crystal. In this position, the tubes receive photons; however, nonuniformity of the image may result.
Attempts have been made with various degrees of success to electronically correct the nonuniformity which is created when the photomultiplier tubes receive radiation from the crystal assembly. The correction of field uniformity is important to the correct reading of results. That is, if nonuniformity is present, a misinterpretation of clinical test could occur.
An electronic uniformity correction scheme is shown in U.S. Pat. No. 3,745,345 Muehllenhner. In the Muehllenhner patent, correction factors are determined, stored and employed to correct output signals from the camera by changing the x,y coordinates. A wired program data system receives digitized coordinate signals and corrects the digitized signals in accordance with stored coordinate correction factors. The corrected digitized signals are then reconverted to analog form and used to operate a conventional CRT display. The disadvantage of this system is that it does not use the original analog coordinate signals for display purposes and does not operate in real time. By the time the deflection voltages are generated, the positional information has been converted, altered and reconverted, thus, introducing error as well as delay with each operation.